1. Field of the Invention
This invention relates to switched-capacitor circuits in general and, more particularly, to switched-capacitor filters used in high-fidelity applications.
2. Description of the Prior Art
Switched-capacitor (S-C) circuits are used in a variety of applications, such as filters, .DELTA.-.SIGMA. analog-to-digital and digital-to-analog converters, etc. These circuits are readily integratable with other circuits, making them useful in VLSI applications. In particular, the use of S-C circuits for digital audio applications is receiving much attention as a way of reducing the cost of the integrated circuits therein by integrating more functions onto fewer chips. However, S-C circuits as known in the prior art are not best suited for high-fidelity/low-noise applications such as digital audio; the signal distortions and self-generated noise produced by S-C circuit operation can dominate the dynamic range and low noise level that digital audio techniques are capable of.
An exemplary general purpose S-C filter is presented in an article titled Parasitic Insensitive, Biphase Switched Capacitor Filters Realized With One Operational Amplifier Per Pole Pair by K. R. Laker, P. E. Fleischer, and A. Ganesan, Bell System Technical Journal, Vol. 61, No. 5, May/June 1982, pp. 685-707. As shown in FIG. 4a of the above-identified article, capacitor A is switched by two switches, one switch switching between a signal and ground, while the other switch alternately couples capacitor A between the input of amplifier 11 and ground. Capacitor A is shown as being "diagonally" switched, while capacitor C is "through" switched. For purposes here, either type of switching, or a combination of the switching types, may be used in a S-C circuit.
The switching of capacitor A is typically a break-before-make arrangement such that the contacts of the respective switch are not coupled together at any time. This is accomplished by using non-overlapping clocks to drive each portion of the switch coupling between a corresponding contact and a common point thereof. Further, both switches associated with capacitor A (or capacitor C) operate at substantially the same time, usually driven by the same clock signals.
The method of switching the switches at substantially the same time produces two major kinds of distortion. One type of distortion is clock feedthrough and charge injection, where the clock signal driving a switch couples through the switch and corrupts the desired signal. Another kind of distortion is dependent on the signal level, commonly referred to as signal-dependent distortion. The first kind of distortion, the clock feedthrough and charge injection, is addressed in Low-Distortion Switched-Capacitor Filter Design Techniques, by Lee and Meyer, IEEE Journal of Solid-State Circuits, Vol. SC-20, No. 6, Dec. 1985, pp. 1103-1112. In Lee and Meyer,, the individual switches are significantly delayed in their operation such that the switches do not operate simultaneously. To illustrate this switching technique, a simplified S-C circuit, here a low-pass filter, is shown in FIGS. 4a14 4e, of this patent application, for one-half of a complete switching cycle. Note that this simplification removes the differential operation of the Lee and Meyer circuit and is for illustrative purposes. Initially, as shown in FIG. 4a, an exemplary filter 40 has switches 41, 42 coupling capacitor 43 between a first signal source 44 and ground. Then switch 42 enters a neutral position, as shown in FIG. 4b. Next, switch 41 enters a neutral position, as shown in FIG. 4c. Note that capacitor 43 is now not coupled to any signal or node; it is completely "floating". Switch 42 then completes its transition, coupling capacitor 43 to the inverting input of operational amplifier 46, shown in FIG. 4d. Note that the operational amplifier 46 is configured such that the input thereto is at virtual ground by virtue of the non-inverting input thereof connecting to ground and capacitor 47 coupling the output of the amplifier 46 back to the inverting input thereof. Finally, switch 41 completes its transition by coupling capacitor 43 to a second signal source 45, as shown in FIG. 4e. For the filter 40 to return to the initial state shown in FIG. 4a, the above-described steps are repeated but for the different initial positions of the switches 41, 42. This switching method class result in reduced clock feedthrough and charge injection than from the simultaneous switching method, discussed above. Although the signal-dependent distortion is reduced, the reduction is insufficient for low distortion applications.